1. Field of the Invention
The present invention relates to a camera which performs automatic focusing by using image signal information whose contents change with whether the image is in-focus or out-of-focus.
2. Related Background Art
The development in the field of cameras has progressed remarkably in recent years. The improvement on the quality of cameras takes an important role, as for example, an automatic focusing device is mounted on each camera as a standard device.
In a VTR built-in type camera having a television camera and a video tape recorder, optical signals passed through the optical system including lenses of the camera unit are converted into electrical signals by image pick-up elements and recorded by the tape recorder. It is a general tendency that the electrical signals are also used in automatic focusing (hereinafter called "AF" where applicable), to use mechanical and electrical components in common and reduce the volume and weight of the camera.
FIG. 1 shows an optical system of a VTR built-in type camera. Reference numeral 101 represents a first lens group for focusing (hereinafter called "focus lens" where applicable). Reference numeral 102 represents a second lens group for variable magnification (hereinafter called "zoom lens" where applicable). Reference numeral 103 represents a third lens group for adjusting the focal plane which changes with a movement of the zoom lens 102. Reference numeral 104 represents an aperture, and reference numeral 105 represents a fourth lens group for correctly focusing an optical image on an image pick-up surface of an image pick-up device 106. The optical image information received at the image pick-up surface 106a of the image pick-up device 106 is converted into electrical image signals. The first lens group 101 moves in parallel with an optical axis L to focus an image of a subject on the image pick-up surface 106a. It is generally known that the image signal outputted from the image pick-up device 106 has higher frequency components, the more the optical system is set near the in-focus state of the subject. The high frequency component of the image signal can be transformed to a focus voltage FV. The characteristic of a focus voltage FV is shown in FIG. 2.
In FIG. 2, reference numeral 201 represents a curve of the focus voltage FV. Reference numerals 202 and 203 represent arrows indicating a change of the focus voltage while the first lens group 101 reaches an in-focus state from an out-of-focus state. The focus voltage FV becomes maximum at the focusing point Pf, and lowers at other points. At a much blurry point (the remotest point Po from the focusing point Pf), the focus voltage FV becomes minimum and scarcely changes with a movement of the first lens group 101.
The manner how the focus voltage is changed (as indicated by arrows 202 and 203) will be described with reference to the flow chart shown in FIG. 3. This flow chart illustrates a so-called mount climbing AF control method. In the following description, it is assumed that the lens groups 101 to 103 and 105 are controlled by a microcomputer and the like and that the control starts from the out-of-focus state of the focus lens 101.
Referring to FIG. 3, a control program starts at step S101. At step S102 the focus lens 101 is moved in an optional direction. At step S103 it is checked whether the focus voltage FV lowers as the focus lens 101 moves. At step S104 an actuator is moved in the opposite direction, the actuator driving the focus lens 101. Determined at step S105 is the direction of moving the focus lens 101 toward the focusing point. At step S106 the focus lens 101 is moved in the direction determined at step S105. At step S107 it is checked whether the focus voltage rises. At step S108 it is checked whether the focus voltage FV lowers. At step S109 the direction of the actuator driving the focus lens 101 is reversed. At step S110 it is checked whether a change in the focus voltage FV becomes small or ceases. At step S111 it is judged that the focus lens 101 has reached the focusing point, and so the focus lens 101 is stopped. At step S112 the control program terminates.
When the program shown in the flow chart of FIG. 3 starts (S101), the focus lens 101 moves in an optional direction (S102). If it is judged at step S103 that the focus voltage FV lowers (Yes judgment), it means that the focus lens 101 moves in the direction away from the focusing point Pf. Therefore, the focus lens 101 is moved in the opposite direction (S104), and the focus voltage FV is again checked (S103). If it is judged at step S103 that the focus voltage FV does not lower (No judgment), this direction is determined as the direction of the focus lens 101 toward the focusing point Pf (step S105) to then move the focus lens 101 in the determined direction.
While the focus lens 101 moves, it is checked if the focus voltage FV rises (S107). This state is indicated by the arrow 202 shown in FIG. 2.
Assuming that the focus lens 101 has passed the focusing point Pf, the focus voltage FV once reaches the flat top portion of the curve 201 and it will be judged at step S107 that the focus voltage FV does not rise (No judgment). As the focus lens 101 continues to move, the focus voltage starts lowering as indicated by the arrow 203 of FIG. 2. This lowering is checked (S108), and if it is confirmed (Yes judgment), the direction of the actuator driving the focus lens 101 is reversed (S109). When the focus lens 101 again reaches the flat top of the curve 201, it is checked at step S110 whether a change in the focus voltage FV becomes small or ceases. If confirmed (Yes judgment), this point is judged as the focusing point Pf, and the focus lens 101 is stopped (S111).
The method described with FIG. 3 is a fundamental AF method using image signals.
With the above-described method, the discrimination between in-focus and out-of-focus states and the determination of the direction toward the focusing point are made basing upon only a difference between the previous one focus voltage FV and the present one focus voltage FV. This method using such a small amount of information has the following disadvantages.
(1) There is a high possibility of errors of the discrimination between in-focus and out-of-focus states and the determination of the direction toward the focusing point.
(2) It is necessary to use an additional program for solving the disadvantage (1), and hence the control becomes complicated.
(3) The control becomes hard to be converged once a false judgment is made.
(4) It is difficult to estimate the operation of the AF control, resulting in an uncertainty of the control.
(5) It is difficult to deal with factors disturbing the control system, such as noises superimposed upon image signals, an abrupt change in position of a subject to be focused.
(6) The focus voltage FV changes greatly before it takes the in-focus state. However, the focus voltage FV will not change both in the in-focus state and much blurry state. Therefore, it is impossible to discriminate between the in-focus and much blurry states even if a plurality of past changes of the focus voltage FV are monitored and the tendency of their changes can be grasped.
(7) If the focus voltage changes abruptly because a subject quickly moves in, or out of, the range finding frame (in-focus detecting area), a false judgment is likely to occur that an in-focus state has passed during the AF control routine.
(8) If the focus voltage fluctuates because of noises on electrical circuits, it becomes somewhat difficult to discriminate between in-focus and out-of-focus states during the AF control routine.
(9) During the evaluation of the in-focus state by using the absolute values of the focus voltages, an abrupt change in the absolute value to be caused by noises superimposed on image signals, and a subject moving in, or out of, the range finding frame, may disturb the control system thus providing a possible false AF control operation.